Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
In order to supply power to the power grid, wind turbines need to conform to certain requirements. For example, wind turbines may need to offer fault-ride through (e.g. low-voltage ride through) capability, which requires a wind turbine to stay connected to the power grid during one or more grid faults. As used herein, the terms “grid fault,” “fault,” or similar are intended to cover a change in the magnitude of grid voltage for a certain time duration. For example, when a grid fault occurs, voltage in the system can decrease by a significant amount for a short duration (e.g. typically less than 500 milliseconds). In addition, grid faults may occur for a variety of reasons, including but not limited to a phase conductor being connected to ground (i.e. a ground fault), short circuiting between two or more phase conductors, lightning and/or wind storms, and/or a transmission line being connected to the ground by accident.
In the past, during these inadvertent faults, it has been acceptable for a wind turbine to be immediately disconnected whenever the voltage reduction occurs. However, as wind turbines continue to increase in size and penetration of wind turbines on the grid increases, it is desirable for the wind turbines to remain on line and ride through such disturbances. In addition, it is also important for the wind turbines to generate energy after the fault is cleared. While the fault is still present and before recovery, it is beneficial for the wind turbine to supply reactive current to the power grid. Since grid faults are of a short duration, it is reasonable that the response of the reactive current be such that it reaches a specific magnitude within specific time. Thus, grid codes for some countries require a minimum reactive current response time during the onset of a low voltage ride-through (LVRT) event.
In many cases, it may not be optimal for the response time of the fastest current regulators of the power converter to be fast enough to meet the grid code requirements. Thus, it may be advantageous for the controller responsible for reactive current regulation to obtain the required response time by requesting a reactive current magnitude that is of greater magnitude than the code requires for an amount of time that is sufficient to speed up the response time required to reach the required amount of current. Such a reactive current reference command can result in the required current being obtained faster than it would have been without the additional command.
More specifically, the present disclosure provides a transient pulse that can be combined in with the reference reactive current required for the LVRT event in order to achieve a transiently higher total reactive current command. The transiently higher current command improves the current response time of the wind turbine and also prevents an excessive current command that may surpass system capabilities.